Treatment of environmental contamination using sepiolite ...Table 1 Global sepiolite distribution...
Transcript of Treatment of environmental contamination using sepiolite ...Table 1 Global sepiolite distribution...
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ORIGINAL PAPER
Treatment of environmental contamination using sepiolite:current approaches and future potential
Na Song . Andrew Hursthouse . Iain McLellan . Zhenghua Wang
Received: 15 December 2019 / Accepted: 25 August 2020
� The Author(s) 2020
Abstract To evaluate the potential of sepiolite-
based materials to resolve environmental pollution
problems, a study is needed which looks at the whole
life cycle of material application, including the
residual value of material classified as waste from
the exploitation of sepiolite deposits in the region or
from its processing and purification. This would also
maximize value from the exploitation process and
provide new potential for local waste management.
We review the geographical distribution of sepiolite,
its application in the treatment of potentially toxic
elements in soil and across the wider landscape, an
assessment of modification and compositional varia-
tion of sepiolite-based applications within site reme-
diation and wastewater treatment. The potential of
sepiolite-based technologies is widespread and a
number of processes utilize sepiolite-derived materi-
als. Along with its intrinsic characteristics, both the
long-term durability and the cost-effectiveness of the
application need to be considered, making it possible
to design ready-to-use products with good market
acceptance. From a critical analysis of the literature,
the most frequently associated terms associated with
sepiolite powder are the use of lime and bentonite,
while fly ash ranked in the top ten of the most
frequently used material with sepiolite. These add
improved performance for the inclusion as a soil or
wastewater treatment options, alone or applied in
combination with other treatment methods. This
approach needs an integrated assessment to establish
economic viability and environmental performance.
Applications are not commonly evaluated from a cost–
benefit perspective, in particular in relation to case
studies within geographical regions hosting primary
sepiolite deposits and wastes that have the potential for
beneficial reuse.
Keywords Modified sepiolite � Adsorption �Potentially toxic elements � Remediation materialcomposition � Economic analysis
Introduction
Environmental contamination is a persistent issue
bringing more challenges to the health and develop-
ment of the world (Uddin 2017). Pollution caused by
the excessive presence of potentially toxic elements
(PTEs) is one of the common environmental issues
and relates to soil, water, sediment, and air. The
magnitude of the total release is difficult to accurately
N. Song (&) � A. Hursthouse � I. McLellanSchool of Computing, Engineering and Physical Sciences,
University of the West of Scotland, Paisley PA1 2BE, UK
e-mail: [email protected]
A. Hursthouse � Z. WangHunan Provincial Key Laboratory of Shale Gas Resource
Utilization, Hunan University of Science and Technology,
Xiangtan 411201, China
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estimate given the diverse point and diffuse sources
associated with human activity. However, it is
reported that an estimated 8.2% of investigated arable
land (7.59 million ha) in China is contaminated with
PTEs due to (i) high natural concentrations (local
geology), (ii) secondary enrichment in soil-forming
process and (iii) human activities (China Geological
Survey 2015). The environmental problems caused by
PTEs are not only affected by their concentrations but
also by the different chemical species present under
varying environmental conditions. The persistence of
PTEs in the environment is prolonged because of
resuspension and circulation in the atmosphere and
transfer between terrestrial and aquatic systems (Sho-
tyk et al. 2003). This is particularly true in areas of
mining, ore exploitation, and metallurgical activity
where in addition to direct soil deposition, rivers and
wetlands have been contaminated by the PTEs. In
addition to the PTEs content, hazards from PTEs are
also affected by environmental conditions. In areas
where soil PTEs exceed regulatory standards due to
high geological background without human activity,
the impact from soil PTEs is relatively low and
transfer in the food chain is not significant. It is easier
for crops to absorb and accumulate PTEs in soils that
have been introduced through human activity, making
these sources, in general, more accessible to wider
dispersal due to chemical reactivity and bioavailability
(China Geological Survey 2015). With the changes of
soil environmental conditions, a high pollution level of
soils with PTEs may lead to migration of contaminants
into adjacent areas, which consequently generate
hazard to the environment and living organisms
(Twaróg et al. 2020). High quantities of PTEs existed
in the wastewater from mining, chemical processing
plants. The contamination was discharged into the
environment as stable forms. Where PTE ions are
stable in wastewater or other contaminated aqueous
environments, they can be ingested and accumulated
easily by aquatic and terrestrial organisms as they are
not biodegradable, with the potential to threaten
human health through transfer in the food chain (Ajao
et al. 2020).
In the management of industrial wastewater, atten-
tion has been paid to the treatment of PTEs, such as
Cu, Zn, Pb, Cd, Hg, As, Cr and Ni (Ihsanullah et al.
2016) as these are the PTEs more strongly influenced
by human activity. Many physical, chemical and
biological techniques have been applied to remove
PTEs from wastewater including chemical precipita-
tion, physical adsorption, electrolysis, biological
accumulation, reverse osmosis and ion exchange
(Ihsanullah et al. 2016). Adsorption is one of the most
viable methods for pollution treatment, owning to its
relatively straightforward application, high efficiency
and insensitivity to the toxicities of contaminants.
Activated carbon is probably one of the most widely
applied adsorbents with many examples of application
to remediate contaminated soil (Sörengård et al.
2019), sediment (Rakowska et al. 2014), atmosphere
(Huang et al. 2019) and water (Hadi et al. 2015). More
novel carbonaceous materials have also been studied
for application in land/soil/sediment treatment, includ-
ing a wide range of nanomaterials (Shahamirifard
et al. 2016), such as graphene oxide (Isari et al. 2018;
Zhao et al. 2011) and carbon nanotubes (Tofighy and
Mohammadi 2015). However, these systems often
require extensive synthesis, purification, and as viable
treatment technologies still require further optimiza-
tion. In pursuit of solutions within the life cycle
approach to reduce the environmental impact of
technologies, more inexpensive and low-cost materi-
als still need to be sought.
Currently, clay minerals are a matter of much
attention because they have been utilized as effective
amendments for soil contaminated with PTEs and
low-cost adsorbents for removing PTEs ions in
aqueous solution (Uddin 2017; Xu et al. 2017). As a
commonly used industrial material, sepiolite has been
used as a therapeutic, functional, inert and bulk agent
in the pharmaceutical and cosmetic industries (Car-
retero and Pozo 2010) and as an adsorbent to reduce
the bitterness of cold-pressed grapefruit seed oil
(Guneser and Yilmaz 2017). It has also been applied
in the sorption of PTEs to improve environmental
conditions (Álvarez-Ayuso and Garcı́a-Sánchez 2003;
Kocaoba 2009; Lazarević et al. 2007).
Sepiolite [Mg8Si12O30(OH)4�8H2O] is a complexhydrated magnesium silicate with a porous fibrous
structure composed of two blocks of tetrahedral silica
sheets and a sandwich octahedral sheet of magnesium
hydroxide or oxide (Xu et al. 2017). Sepiolite is
widespread and abundant in the earth and most
research focuses on the application of natural sepiolite
or purified products to remove pollutants from the
environment. In addition, applications of purified, or
modified, sepiolite are found in improving mechanical
properties of materials, improvement of
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environmental quality (e.g. air) and in craftwork.
There is also an increased interest in modified sepiolite
materials for environmental treatment, exploiting
higher performance characteristics that can be
obtained (Padilla-Ortega et al. 2013).
The extraction and purification of sepiolite require
energy input and with the current focus on the climate
emergency, the process alone does not meet full
circular economy objectives. To address these, the
potential of resolving pollution problems and main-
taining a low carbon footprint, a study is required that
considers the whole life cycle including maximizing
the residual value of material classified as waste
materials from the extraction and the purification of
sepiolite to provide more environmentally friendly
and energy-saving methods for contamination treat-
ment. We have previously considered the context of
sepiolite as a potential technology to treat localized
contamination in China (Wang et al. 2018), highlight-
ing capacity and treatment opportunities for PTEs in
aquatic systems, modification and regeneration issues.
This review provides a perspective on the life cycle of
technology considering the potential application of the
raw material, exploitation of residues, properties, and
possible risk with the application of sepiolite in
projects for the treatment of contaminated soil or land.
Geographical distribution of sepiolite
Suitability of structural features and properties of
sepiolite make them very versatile materials that can
be applied in many industries and thereby essential
and crucial for the world economy as a whole. Spain
is the world’s largest producer of sepiolite. In Europe,
90% of sepiolite comes from Spain, and the rest of the
sepiolite comes mainly from Turkey. Tblsa SA is the
largest producer of sepiolite in Spain with a produc-
tion capacity of 700,000 tons/year, providing 515,000
tons of sepiolite products: of which about 353,000
tons are used in cat and pet litters, and about 162,000
tons are consumed as industrial adsorbents, carriers
for chemicals (e.g. pesticides), animal feedstuffs (Yi
2003). The price of sepiolite products depends on
particle size, degree of modification, taste, additives,
treatment methods. Sepiolite prices vary depending
on the degree of processing, as a guide, typical
examples from the Spanish markets give low-grade
processed sepiolite products * $32/t, industrial
products * $112/t, rheology modifier prod-ucts * $934/t, and specialized modified sepioliteproducts * $1871/t (Yi 2003). Raw sepiolite pricesfrom China and Turkey are $180–300/t and $500–800/
t, respectively, on Alibaba website: (https://www.
alibaba.com/trade/search?fsb=y&IndexArea=product_
en&CatId=&SearchText=raw?sepiolite&SearchSce
ne=exhibition&spm=a27aq.14827689).
China hosts one of the world’s major sepiolite
reserves (Yin et al. 2011). The wide applicability and
utility of sepiolite in the industry for rheological and
catalytic purposes is well known and has driven effort
for industrial exploitation for over 200 years (Martin
Vivaldi and Robertson 1971). There is a potential in
sepiolite-based coating material that helps with arrest-
ing heat transmission to reduce energy consumption in
daily lives (Wang et al. 2009). Deposits of Hunan
province comprise weathered Marine-sedimentary
formations, consisting of sepiolite-palygorskite beds
in the upper layer of the Permian Qixia (Chihsia)
formation and have provided raw materials for purifi-
cation of high-quality sepiolite products for industrial
application. Information on the global distribution of
sepiolite deposits is given in Table 1. The chemical
composition of the range of sepiolite sources is given
in Table 2.
Sepiolite-based material single application
Polymer nanocomposite sepiolite
Application in machinery
Sepiolite has remarkable adsorptive properties as a
result of high surface area, and therefore it has been
widely used in the preparation of polymer nanocom-
posites preparation as it increases dimensional stabil-
ity, processability, thermal resistance and mechanical
strength. The influence of different nanofillers, such as
sepiolite, has been studied on the properties and
characteristics of natural rubber nanocomposites,
which showed the maximum improvement in modulus
because of the high surface energy of sepiolite which
is 38 mJ/m2 (Bhattacharya et al. 2009).
The effect of sepiolite nanoclay on the tack strength
of brominated isobutylene-co-p-methyl styrene loaded
rubber was studied (Kumar et al. 2010). It was
revealed that tack strength increased by around
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https://www.alibaba.com/trade/search%3ffsb%3dy%26IndexArea%3dproduct_en%26CatId%3d%26SearchText%3draw%2bsepiolite%26SearchScene%3dexhibition%26spm%3da27aq.14827689https://www.alibaba.com/trade/search%3ffsb%3dy%26IndexArea%3dproduct_en%26CatId%3d%26SearchText%3draw%2bsepiolite%26SearchScene%3dexhibition%26spm%3da27aq.14827689https://www.alibaba.com/trade/search%3ffsb%3dy%26IndexArea%3dproduct_en%26CatId%3d%26SearchText%3draw%2bsepiolite%26SearchScene%3dexhibition%26spm%3da27aq.14827689https://www.alibaba.com/trade/search%3ffsb%3dy%26IndexArea%3dproduct_en%26CatId%3d%26SearchText%3draw%2bsepiolite%26SearchScene%3dexhibition%26spm%3da27aq.14827689
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Table
1Global
sepiolite
distribution
Distribution
Country
Deposit
Depositiontype
Estim
ated
reserve(ton)
Approxim
atelyannual
output
(ton)
Study
1NorthAmerica
USA
Amargosa,Nevada
––
10000(inGeorgia)
(Eberlet
al.1982;Khoury
etal.1982;Liang2008)
2Europe
Spain
Vallecas-Vica9 lvaro-
Yunclillos,Batallones
andMara
––
700,000
(Mayayoet
al.1998;Yi
2003)
3Western
Asia
Turkey
Eskisehir
Marinesepiolite-
palygorskite
17,000(nodularsepiolite)for
Eskisehir
20,000–60,000(in1990s)
forTurkey
(DPT2001;Hüseyin
and
Bozkaya2011;Yalçinand
Bozkaya1995)
Lacustrinesepiolite-
palygorskite
(YalçinandBozkaya1995)
Hydrothermal
sepiolite
(İrkec
andÜnlü
1993)
Hydrothermal
sepiolite
(YalçinandBozkaya2011)
Anatolia
–8.5
millionforcentral
Anatolia
(Hüseyin
andBozkaya2011)
4EastAsia
China
Hunan,Henan
–20millionforHunan
–(W
angetal.2018;Zhangand
Yang1986)
5EastAfrica
Kenya-
Tanzania
Amboseli
––
320
(Liang2008;Stoesselland
Hay
1978)
6Africa
Somalia
EI-Bur
––
Hundreds
(Liang2008;Singer
etal.
1998)
7Thewhole
world
–850,000
(Galán
andPozo
2015)
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Table
2Sepiolite
compositionacross
theworld
SiO
2TiO
2Al 2O3
Fe 2O3
FeO
MnO
CaO
MgO
K2O
Na 2O
H2O?
H2O-
LOI
Total
Study
159.41
0.02
0.42
0.76
0.02
0.08
1.47
24.03
0.17
0.05
9.22
9.68
105.33
(ZhangandYang1986)
260.88
0.002
0.45
0.48
0.12
0.004
0.08
25.68
00.03
11.68
9.5*
99.41
(IGGCMBMI1980)
358.71
0.02
0.9
1.07
0.09
0.06
1.86
25.06
0.1
0.35
9.75
99.43
(ZhangandYang1986)
461.4
–0.38
0.19
0.07
–0.79
26.51
\0.01
0.03
11.17
9.45*
100.55
(IGGCMBMI1980)
552.69
0.03
0.41
0.44
0.2
0.1
7.2
24.29
––
8.35
14.51
99.8
(ZhangandYang1986)
655.10
0.05
3.13
0.04
0.01
0.04
0.23
22.37
0.16
0.08
7.40
11.40
100.01
(Peng2017)
754.44
–0.4
0.15
0–
0.11
24.59
00.05
9.12
11.5
100.39
(ZhangandYang1986)
854.75
–0.8
4.1
––
1.68
18.3
20
(Liu
andXu1984)
955.21
0.05
0.43
0.15
0.02
0.2
24.26
0.15
0.1
––
19.21
99.78
(Lopez
Galindoet
al.1996)
10
63.10
–1.08
0.27
––
0.49
23.80
0.21
0.09
10.88
––
99.92
(Galan
andCastillo1984)
11
53.17
0.17
1.76
0.99
0.04
–0.23
24.70
0.97
0.45
8.29
8.79
–99.57
(Hay
andStoessell1984)
12
56.46
0.08
1.17
0.37
––
0.19
22.67
0.18
0.06
––
18.57
(Pozo
etal.2014)
13
56.95
0.15
1.05
0.93
––
2.45
23.35
0.41
0.11
––
14.60
(Ece
andÇoban
1994)
*Thetotalam
ountdoes
notincludeH2O-
Fiberssepiolite
(a-type):(1)Henan
Neixiang;(2)Henan
Lushi;(3)AnhuiQuanjiao;(4)Hubei
Guangji;(5)ZhejiangLin’an;(6)Hunan
Yonghe;
(7)Sichuan
Shim
ian;(8)
GuangxiDu’an;(13)Turkey
Eskişehir
Claysepiolite
(b-type):(9)SpainMadrid;(10)SpainVallecas;
(11)Kenia
Amboseli;(12)SpainBatallones;(13)Turkey
Eskişehir
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300% over that of neat BIMS (brominated isobuty-
lene-co-p-methyl styrene) rubber. The molecular
diffusion across the interface of BIMS rubber was
lowered upon the addition of sepiolite nanoclay due to
its reinforcing effect. However, the average penetra-
tion depth of the diffused chains of BIMS in the
composites was still sufficient to form entanglements
at either side of the interfaces, leading to greater
resistance to separation due to an increase in cohesive
strength of the corresponding nanocomposites (Kumar
et al. 2010).
Sepiolite was modified by Tartaglione et al. (2008)
in two different processes of functionalization: surface
adsorption of quaternary ammonium salts/amines or
grafting of silane reagents to surface silanols by
covalent bonding. The grafted sepiolite-based
nanocomposites of poly (butylene terephthalate)
(PBT) and polypropylene (PP) are performed higher
thermal stability compared to those of adsorbed
sepiolite. There is no effect of sepiolite on thermal
characters of PBT, whereas there is a polymer
crystalline structure revealed on PP nanocomposites.
Sepiolite was modified with cetyltrimethyl ammo-
nium bromide (CTAB) by ion exchange and further
was used to prepare polypropylene and polypropylene
grafted with maleic anhydride composites by melting
extrusion. The properties of modified sepiolite were
compared with those of unmodified sepiolite-based
nanocomposites. It is sepiolite that was modified by
polypropylene/polypropylene grafted maleic anhy-
dride/CTAB, showing the best interaction and result-
ing in increased elastic modulus and thermal
resistance. There is a lower maximum heat release
rate for the composite than those of neat polymer.
Application in wastewater
In wastewater treatment, sepiolite has been applied as
a denser core anchoring chain for a polyelectrolyte,
such as a polycationic polymer, with charge opposed
to the colloidal charges, which leads to the fast
formation of neutralized flocs with a higher density
than organic colloids (Rytwo 2014, 2017). Sepiolite
polymer nanocomposites offer effective clarification
of water with a reduction of turbidity and suspended
solids by more than 90% in effluents (Rytwo
2014, 2017). It is a technology that could be developed
for wide ranges of applications. The properties of the
polymer will be combined with that of the fillers,
which means the significance of modification is
obvious.
A novel amidoxime based chelating nanohybrid
adsorbent was synthesized with poly (acrylonitrile)
grafted sepiolite nanohybrid material (RGS) with
simultaneous radiation grafting technique (Taimur
et al. 2017). Batch adsorption studies were carried out
for Cu adsorbed on amidoximated nanohybrid adsor-
bents to study the effects of pH, contact time,
adsorbent dose and initial concentration. Removal of
Cu ion attained equilibrium follows pseudo-second-
order kinetics within 30 min. There is a good
description of the equilibrium process by the Lang-
muir isotherm model, and adsorption capacity reached
its maximum at 278 mg/g for irradiated samples
(Taimur et al. 2017).
Polyaniline/sepiolite (PANI/sepiolite) nanofibers
were synthesized by chemical oxidation polymeriza-
tion to remove Cr6? from aqueous solution (Chen et al.
2014). The adsorption of Cr6? to the PANI/sepiolite
nanofibers was strongly influenced by pH within a
range of 2–10, and adsorption efficiency of Cr6? to the
PANI/sepiolite reached the highest at pH 2. The
maximum adsorption capacity of the nanofibers for
Cr6? was utmost at 206.6 mg/g at 25 �C, and itincreased with the increase in temperature. Experi-
ments of desorption revealed that PANI/sepiolite can
be reused and regenerated for 2 continuous cycles
without loss of its removal efficiency (100% in the first
cycle and removal efficiency remained almost the
same in the second cycle), which indicated that the
nanofibers can be used as an economical and efficient
adsorbent for Cr6? (Chen et al. 2014).
Modified sepiolite combines with other fillers has
been a subject of research and the work has been
started. The opportunities for sepiolite applications are
vast and hence there are numerous opportunities to
utilize sepiolite. Along with its specific characteristics,
both the long-term durability and the cost-effective-
ness of the application need to be considered, making
it possible to design ready-to-use products with good
market acceptance. Sepiolite polymer nanocomposite
has not been applied in the treatment of polluted soil or
land so far because it is difficult and expensive to
retrieve the polymer nanocomposite for reprocessing/
reuse.
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Other modified sepiolite materials for treatment
Application in water
To improve PTEs adsorption to sepiolite, there are a
number of modification methods, including acid
treatment, thermal treatment, magnetic modification,
organic-modification and acid-thermal treatment
(Wang et al. 2018). A method for removal of PTEs
using nanoscale-zero-valent iron material loaded with
sepiolite in aquaculture water involves microwave-
assisted acid modification of sepiolite powder in
hydrochloric acid solution. Therefore, it can quickly
adsorb and remove PTEs from the aquaculture water
body, help to adjust the pH to meet the requirements
for good aquaculture conditions. The reaction condi-
tions required are relatively easily achieved, the raw
materials are low cost and easy to obtain, and this
method is overall less time-consuming compared with
traditional chemical precipitation of PTEs. The reac-
tion is mild and the method is suitable for wider
operation (Liu et al. 2018). In addition, to treat Cu in
aquaculture wastewater, negatively magnetically
modified sepiolite has been used to adsorb the Cu
ions followed by filtering the sepiolite adsorbent and
post-treatment processing (Zhang et al. 2018). A
comparison of the adsorption capacity of waste
sepiolite, modified sepiolite, natural sepiolite and
other materials is given in Table 3.
Studies have been carried out on the structural
effect of cationic surfactant to treatment capacity of
PTEs (Kalpakli and Cansev 2018). The organo-clays
were synthesized with smectite and hexade-
cyltrimethylammonium bromide (HDTMAB) which
is a cationic surfactant. Moreover, the characteristics
of prepared organoclay to remove Cd2?, Cu2? and
Pb2? ions from solution have been studied and the
results demonstrated that organoclay increased their
adsorption capacity for the Pb, Cu and Cd from
aqueous solution (Kalpakli and Cansev 2018). A
sepiolite modified material was prepared by micro-
wave reaction of the mixture of sepiolite and kaolin
and the mixture of ferric salt of cellulose, ferrous
chloride and ferric chloride and then applied to remove
Cd2?, Cu2?, Cr6? and Pb2? ions from solution with
the benefit of a lower cost, higher adsorption efficiency
and stability of the material (Huang 2017).
Composites of natural sepiolite (SEP) and partially
acid-activated (AAS) were prepared at different ratios
and nanoscale zero-valent iron (nZVI) to obtain the
best nZVI dispersibility and best performance of
adsorption of Cd2? (Habish et al. 2017). Though there
is a higher surface area and pore volume of nZVI
composed with AAS, the dispersibility of nZVI was
better when SEP was used as the support. However, a
lower degree of oxidation was attained during the
synthesis by AAS. X-ray photoelectron spectroscopy
(XPS) analysis of the composite with the best
dispersibility for nZVI before and after adsorption of
Cd2?, indicated that the surface of the nZVI particles
was composed of oxidized iron species. The adsorp-
tion isotherms of Cd2? revealed that the existence of
SEP and AAS reduced the agglomeration of nZVI
particles when being compared to the pure nZVI,
which showed higher adsorption capacity (Habish
et al. 2017). A study was carried out by Bahabadi et al.
(2017) on the application of natural sepiolite and Fe3?
modified sepiolite for the treatment of Cu, Zn and Cd
in aqueous solutions. The concentration of iron
chloride applied for modification was 1 M. Zn
removal by sepiolite remained the same before and
after the surface modification, while removal of Cu
and Cd were increased by 13.6% and 21.2%,
respectively.
A modified sepiolite was prepared with magnetite
by chemical co-precipitation, and the magnetite/sepi-
olite (M/SEP) composite was characterized by X-ray
diffraction (XRD), scanning electron microscopy
(SEM), Fourier transform infrared spectroscopy (FT-
IR), vibrating sample magnetometer (VSM) and X-ray
photoelectron spectroscopy (XPS). The results
revealed the composition of M/SEP is magnetite and
sepiolite, having a superparamagnetic property. The
results of the application ofM/SEP to adsorb Co2? and
Cd2? from aqueous solutions showed that sorption
processes were endothermic and spontaneous pro-
cesses on the basis of thermodynamic parameters,
which indicates that M/SEP composite can be used on
the preconcentration of Co2? and Cd2? in wastewater
treatment (Yu et al. 2016).
Furthermore, sepiolite was also modified by N-1-
[3-(trimethoxysilylpropyl)] and the interaction with
PTEs was studied systematically as a function of ionic
strength, pH and temperature using ASS. The results
revealed that the amount of adsorbed PTEs increased
with an increase in equilibrium pH and temperature of
the solution, while it decreased generally with the
increase in ionic strength. The adsorption mechanism
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Table
3Theadsorptioncapacityofsepiolite,modified
sepiolite
andother
materials
Material
Treatment
PTE(s)
Capacity
Reference
Waste
sepiolite
–Boron
96.15mg/g
(Öztürk
andKavak
2004)
Activated
waste
sepiolite
HClactivated
Boron
178.57mg/g
(Öztürk
andKavak
2004)
Naturalsepiolite
U34.61mg/g
(Donat
2009)
Modified
sepiolite
Modified
by3-(trim
ethoxysilyl)propyl
methacrylate
(monolayer
maxim
um
adsorptioncapacity)
Cu,Mn,
Zn,Fe,
Co,Cd
12.3
910-5mol/L,11.7
910-5mol/L,9.0
910-5
mol/L,8.2
910-5mol/L,5.7
910-5mol/L,
1.8
910-5mol/L
(Beyliet
al.2016a)
Modified
sepiolite
Modified
byN-1-[3-
(trimethoxysilylpropyl)]
Co,Cd,
Mn,Fe,
Cu,Zn
30mol/g,37mol/g,80mol/g,110mol/g,118mol/
g,118mol/g
(Beyliet
al.2016b)
Activated
carbon
Cd,Cr,
Cu,Pb
0.19mg/g,0.16mg/g,0.21mg/g
0.18mg/g
(Sajjadet
al.2017)
Silver
birch
(B.
pen
du
la)biochar
Cu,Pb
0.13mg/g,1.299
10-3–3.779
10-3mg/g
(KomkieneandBaltrenaite
2016)
Scotspine(P
.sy
lves
tris
L.)biochar
Zn,Pb
0.11mg/g,2.379
10-3–4.499
10-3mg/g
(KomkieneandBaltrenaite
2016)
Chitosan
As
50.0
mg/g
(Brion-robyet
al.2018)
Ricestraw
Cr
34.90mg/g
(Lin
etal.2018)
Chineseloess
Ni
15.61mg/g
(Wanget
al.2015)
Modified
agricultural
wastes
Cd
195.5
mg/g
(AlOthman
etal.2011)
123
Environ Geochem Health
-
of metal ions to modified sepiolite was identified by
analyzing the experimental data on the basis of
Langmuir and Freundlich isotherms, which indicated
that the isotherm data were correlated to Langmuir
isotherm. The monolayer adsorption capacities of
modified sepiolite were estimated as 30, 37, 80, 110,
118 and 118 mol/g for Co2?, Cd2?, Mn2?, Fe3?, Cu2?
and Zn2?, respectively. The affinity of adsorption was
in the order of Co2?\Cd2?\Mn2?\ Fe3?-
\Cu2? = Zn2?. The results demonstrated thatCo2? and Cd2? were less efficiently adsorbed than
Fe3?, Cu2? and Zn2? by modified sepiolite. In
conclusion, modified sepiolite can be applied as an
alternative low-cost adsorbent in adsorption process in
nanocomposite synthesis and chromatographic purifi-
cation process (Beyli et al. 2016b).
Sepiolite was synthesized by nanoscale zero-valent
iron particles (S-NZVI) for the adsorption and reduc-
tion of Cr6? and Pb2? ions from groundwater and
results showed that S-NZVI had the potential for
in situ remediation of heavy metal-contaminated
groundwater (Fu et al. 2015). Acid-activated sepiolites
and natural sepiolites were functionalized with cova-
lent grafting [3-(2-aminoethylamino) propyl] tri-
methoxy-silane on the sepiolite surface, and their
adsorption capacities of amine-functionalized sepio-
lites to Cr6? from aqueous solution were investigated
(Marjanović et al. 2013). The adsorption isotherms
showed that the adsorption capacities of the modified
acid activated sepiolite were higher than those of
modified natural sepiolite for all of the researched
initial pH values. The maximum concentrations of
Cr6? removal were 60 mg/g of functionalized acid-
activated sepiolites and 37 mg/g of functionalized
sepiolites, respectively, achieved at an initial pH of 2.0
(Marjanović et al. 2013).
Application in soil
A mercapto functionalized sepiolite material (MSEP)
was prepared as an immobilization agent for treatment
of Cd-polluted paddy soil (Liang et al. 2017). The Cd
content of husked rice (Oryza sativa L.) was reduced
by 65.4–77.9% at the trace dosages of 0.1–0.3% for
MSEP. Single and sequential procedures showed that
MSEP could boost the sorption or fixation of Cd on
soil and decrease the bioavailability of Cd (Liang et al.
2017). MSEP had negligible effects on the point of
zero charge of the paddy soil and pH values. There is
no obvious effect on extractable Zn, available phos-
phorus and hydrolysable nitrogen in the soil, which
indicated that MSEP is environment friendly and
compatible to use (Liang et al. 2017).
All of the sepiolite-based material might have a
good effect on soil remediation; however, reuse or
recycle of products is a problem. Furthermore, it might
be a potential threat that some chemicals of the
products could enter into groundwater.
Sepiolite combined with other additives
in treatment
Raw sepiolite, modified sepiolite and sepiolite tailings
have been applied as an additive combined with other
additives together, which is adsorbent for water
treatment, amendment agent, conditioner agent and
inactivator for soil remediation. For instance, for PTEs
polluted soil application of sepiolite-based material
has included adding 10–385 wt% modified sepiolite
tailings, 1–80 wt% modified zeolite, 1–5 wt% hypo-
sulfite, 10–20 wt% nano iron powder and 45–60 wt%
quicklime (Wang et al. 2017).
We identified 150 sepiolite-based literature reports
and patents from databases (Web of Science, United
States Patent and Trademark Office (https://www.
uspto.gov), Chinese Patent database (http://www.
soopat.com/) for application in the treatment of
water or soil contaminated by PTEs. The word cloud
picture (Fig. 1) is produced by software Nvivo12 with
an analysis of key content of combinations of mate-
rials. Figures 2, 3 and 4 are produced from Microsoft
Office 2010 Excel by frequencies analysis to all
additives of combinations of materials. All of them get
checked if they (natural sepiolite, modified sepiolite,
sepiolite tailings) are used as an additive to combine
other additives for PTEs treatment in water, soil, land,
sludge. There are published papers and patents (Eng-
lish abstract from different countries) in the database
Web of Science. Only published patents and licensed
patents are shown in the US patent office (in English)
and Chinese patent database (in Chinese). A patent is
only counted as one though this one might be pub-
lished and licensed in more than one document.
To synthesize a large amount of literature and
patents published on sepiolite, a word cloud picture
has been prepared (Fig. 1). Figures 2, 3 and 4 describe
the results in more detail.
123
Environ Geochem Health
https://www.uspto.govhttps://www.uspto.govhttp://www.soopat.com/http://www.soopat.com/
-
A total of 10 materials appear with the frequency of
application between 16 and 140 times used in the
formation of composites with sepiolite, followed by
lime (31), bentonite (27), chitosan based (25), zeolite
based (25), activated carbon (20), modified sepiolite
(19), calcium source (19) and straw based (18)
(Fig. 2). We identified 16 materials with a frequency
of application between 10 and 16, the highest being fly
ash (15), followed by phosphate-based (14), modified
iron or iron (13), while potassium-based, polyacry-
lamide, clay mineral, water and humic acid showed the
same frequency (12), followed by starch, biochar,
calcium–magnesium phosphate, diatomite and cellu-
lose-based (11), montmorillonite, kaolin and shell (10)
(Fig. 3). There are 24 materials in the frequencies of
5–9, with the highest frequency is silica-based (9),
followed by humate-based (7), while carbonate-based,
limestone, peat and polyvinyl alcohol-based were at
the same frequency (8), followed by fruit peel, urea
and zinc sulfate (6), charcoal-based, aluminum-based,
ammonium sulfate, vermiculite, manure, alumina,
cattle manure biochar, ferrous sulfate, glass beads,
sodium alginate, cement, gamma-alumina, seaweed,
palygorskite and polyethylene glycol (5). In addition,
the frequencies of 1–4 from all other kinds of materials
are included in the other figure (Fig. 4).
Fly ash or coal fly ash is an industrial by-product
generated from the combustion of coal for thermal
power generation and other combustion activities
(Ahmaruzzaman 2010). In addition, bottom ash and
lagooned ash are formed during the combustion of
coal. Bottom ash shows a similar phase, particle
morphology and chemical composition with fly ash
though the unburnt coal component and proportion of
amorphous particle is greater than that of fly ash.
Lagooned ash shows an intermediate, particle mor-
phology and phase between fly ash and bottom ash
(Vassilev 1992). There are mainly two classes of fly
ash, class F contains more than 8% CaO and class C
contains less than 8% CaO. Both contain various
oxides Al2O3, SiO2, Fe2O3, Na2O, MgO, MnO, P2O5and TiO2. Other elements (Cu, Pb, Cd, Cr) and
radioactive isotopes (232Th, 238U, 40K) can be also
found in some fly ash (Wang et al. 2020).
Emissions and accumulation of fly ash not only
occupy land, but also cause serious pollution to the
Fig. 1 A picture of wordcloud to show materials
appeared in proportion for
PTEs treatment
123
Environ Geochem Health
-
Fig. 3 Variation of additives in sepiolite-based composites (in literature reports 1864–2018)
Fig. 2 Variation of additives in sepiolite-based composites (in literature reports 1864–2018)
123
Environ Geochem Health
-
environment and poses a disposal challenge (Ochedi
et al. 2020). However, fly ash is also a resource that
can be widely applied in the fields of construction
materials, road sub-base, mine backfill etc. (Ahmaruz-
zaman 2010), and fly ash also shows a promising
ability as a heterogeneous catalyst to oxidize aqueous
Fig. 4 Variation of additives in sepiolite-based composites (in literature reports 1864–2018)
123
Environ Geochem Health
-
Na2S solution (Mallik and Chaudhuri 1999) or sorbent
to remove various air pollution (Ahmaruzzaman and
Gupta 2012; Ochedi et al. 2020). Fly ash is utilized as a
constituent of mixtures for solidification/stabilization
technologies because of its low-cost, good pozzolanic
properties, which helps with solid waste disposal and
coping with potential stress of its large generation and
accumulation in an economically beneficial way
(Terzano et al. 2005).
The influence of fly ash on properties of sepiolite is
carried out by adding fly ash to the sepiolite with 2%,
4% and 6% by weight (Joseph and Angel 2020). The
results showed that the pH of the mixture is decreased
with a content increase in fly ash. The specific gravity
and the Atterberg limit of the mixture showed a
negative trend with an additional increase in fly ash.
The mechanical standard compaction test results
revealed a slight increase in the maximum dry density
and a decrease in the optimum moisture content with
an increase percentage of fly ash in the mixture. The
strength and permeability of the mixture were
improved up to 4% of the fly ash content, after that
they showed a decrease in trend. The maximum values
of strength and permeability of the sepiolite were
reached at 4% addition of fly ash (Joseph and Angel
2020).
All of the sepiolite-based material additives might
have a good effect on water treatment which is
recyclable and low cost. They are much safer and
cheaper to be added in soil but could still be more
difficult to control the amount of sepiolite-based
material and combined ratio because of various soil
background comparing with other soil remediation
methods.
Economic constraints for the application
of sepiolite in remediation
Direct project cost and period of treatment
In the research of potential for ensuring the safety of
production of rice planted in Cd-polluted soil by using
sepiolite and humic acid, the production was studied
using Oryza sativa L. (Jia-33) in which the accumu-
lation of Cd with different weights of additives
(including humic acid at 0.750, 2.250, 3.750, 5.250,
7.500 t/ha; or sepiolite at 2.250, 6.750, 11.250, 15.750,
22.500 t/ha, or combination of humic acid and
sepiolite at 0.375 ? 1.125, 1.125 ? 3.375,1.875 ?
5.625,2.625 ? 7.875, 3.750 ? 11.250 t/ha) was
assessed (Xie et al. 2018). Results showed that
applications of humic acid are at 5.250 t/ha, or
sepiolite at 6.750 t/ha or a combination of humic acid
at 1.125 t/ha and sepiolite at 3.375 t/ha leads to that Cd
content of polished rice were 0.171 ± 0.01 mg/kg,
0.184 ± 0.01 mg/kg, and 0.181 ± 0.01 mg/kg,
respectively, lowering than the Cd limit defined in
the National Food Safety Standard (GB 2762-2012,
0.2 mg/kg) of the PRC. The material purchase prices
were 197.59 $/t of humic acid, 112.91 $/t of sepiolite,
which means that the corresponding cost of applica-
tions of 5.250 t/ha humic acid, 6.750 t/ha sepiolite, the
combination of 1.125 t/ha humic acid and 3.375 t/ha
sepiolite were 1037.34, 762.13 and 433.99 $/ha,
respectively (Xie et al. 2018).
Research has shown that Cd passivated by the soil
amendment calcium–magnesium phosphate and lime
starts to re-release from the fourth year, which
revealed that the passivation effect could last for
3 years (Jiang et al. 2017). This is much better than
more intrusive stabilization methods such as lime/
cement as soil function is preserved to some extent.
Regardless of human resource and in terms of the cost
of the materials, the lowest cost for the combination of
humic acid at 1.125 t/ha and sepiolite at 3.375 t/ha
with the average annual cost is about 144.66 $/ha. This
further illustrates the potential of low accumulation
strains of rice combined with the application of the
sepiolite to meet agricultural safety standards (Xie
et al. 2018).
Remediation potential for contaminated land
Sepiolite is applied as amendments or immobilizers
for remediation of soils contaminated with PTEs. The
remediation efficiency and function of sepiolite on the
remediation Pb and Cd-polluted farmland have been
verified with field studies by Zhan et al. (2019). The
pH value of soils increased by 1.67 after the applica-
tion of sepiolite, whereas the CaCl2-extractable con-
centration of Cd and the exchangeable concentration
of Pb in soils decreased by 63.4% and 21.5%,
respectively. The results indicated that immobilization
mechanisms for the sepiolite may be complex, which
not only involved with surface adsorption due to high
specific surface but also involved with a high ion-
exchange capacity of sepiolite (Zhan et al. 2019).
123
Environ Geochem Health
-
Sepiolite materials alone or in a combination with
other materials such as limestone/bentonite/calcium
carbonate can significantly cut down the content of Cd
in brown rice, regardless of its application in a field
study or pot trials (Sun et al. 2016a, b; Wu et al. 2016;
Li et al. 2020). For instance, when sepiolite combined
with low Cd-accumulating cultivar rice (Oryza sativa
L.) in field experiment, the concentration of Cd in
brown rice of low Cd-accumulating cultivar rice
(Oryza sativa L.) from the control group was
0.72 mg�kg-1, while the concentration of Cd in brownrice of the cultivar rice from the experimental group
was 0.18 mg/kg, which is lower than that of the
maximum amount lever proposed as ‘‘Maximum
Levers of Pollutants in Foods’’ standard (GB
2762-2012) in the Chinese national standard and the
Codex Alimentarius Commission (CAC 153-1995)
requirement in theWorld Health Organization (WHO)
(Liang et al. 2014).
However, there are differences in remediation
effects of sepiolite-based amendment on soil polluted
with Cd, Pb. For example, a 3-year in situ experiment
was carried out in a paddy soil around a mine of
southern Hunan, China (Wu et al. 2016). The
combined amendment (LS, limestone and sepiolite)
was applied in order of 0, 2, 4 and 8 g/kg (w/w) and
rice were subsequently planted in 2012 (first season),
2013 (second season) and 2014 (third season). The
results showed that the pH values of soils increased
significantly in all three seasons and the ranking of
enhancement was as follows: the first season[ thesecond season[ the third season. Experiment resultsindicated that the effect of the LS on reducing
exchangeable Pb and Cd content of soil ranked as
follows: the first season (97.6–99.8%)[ the secondseason (80.7–97.7%)[ the third season (32.6–97.7%)for Pb and the first season (88.3–98.9%)[ the secondseason (28.3–88.0%)[ the third season (8.3–71.4%)for Cd, respectively. It showed that the effects were
persistent on soil exchangeable Cd concentration but
gradually decreased with time on soil exchangeable Pb
concentration, which implied that LS was more
suitable for Cd than Pb contaminated soil in long-
term remediation (Wu et al. 2016). Furthermore, a
2-year consecutive experiment was conducted on
immobilization effect of sepiolite alone by Liang
et al. (2016), which revealed that immobilization
effect is maintained seen in the decrease in Cd content
of the rice, 0.025 M HCl extractable Cd content of
paddy soil and the soil exchangeable Cd in the second
year without any additional amendment.
Economic impacts and project risk
In China, contaminated land is identified by the
government or local authority. Typically, several
contamination treatment enterprises will compete via
tender to secure a pollution treatment project. Once a
firm is appointed, a feasibility report for soil remedi-
ation is required for the company to obtain final
approval from the relevant authority. The specific
details of the project are then decided; this involves a
comparison of remediation technologies and methods.
The project would be carried out after the approval is
granted; then immediate or long-term monitoring of
contamination treatment results would be arranged
based on the design.
To use sepiolite for the treatment of land contam-
ination by PTEs will increase the value or quality of
the land and its potential possibility for reuse
commercial purpose, create more space for residential
development in the area, increasing employment in
land remediation and redevelopment, which bring
more local tax revenues (Song et al. 2019). With the
development of society and new knowledge gained by
individuals, the choice of methodology for soil
remediation could be combined with positive interac-
tions with neighborhoods, for example through com-
munity center development as part of the remediation,
which can satisfy the social and economic needs of
different stakeholders. This would support the harmo-
nious integration of new solutions for contamination
treatment within surrounding communities and lead to
a more sustainable local economy (Dagenhart et al.
2006).
When remediation projects involved some related
risk, the regular lifestyle might be impacted, affecting
local traffic movement, environmental degradation
during project implementation, or for workers’ safety
during the remediation period and even neighbor
nuisance complaints. Recent studies of a farmland
remediation project (Xu et al. 2019) showed that by
adopting species sensitivity distribution models with
joint probability curves, a different conclusion was
reached related to ecological risk assessment results
compared to that of the traditional methods. The
conclusion revealed that Cr indicated the highest risk
at 84.3% that presents an 84.3% probability for 5% of
123
Environ Geochem Health
-
the species to exceed NOEC/LOECs (maximum no-
effect concentration and the lowest effective concen-
tration) in Zhuzhou City (Xu et al. 2019). This study
showed that the soil ecological risk of exposure to
PTEs is underestimated by traditional methods and
prioritizing ecosystem protection could be considered
for PTEs contaminated land treatment (Xu et al. 2019).
However, in the site where sepiolite-based material is
applicated, microbial community diversity and soil
enzymatic activities showed that soil metabolic func-
tion is a certain degree recovered and sepiolite
application is environmentally friendly (Sun et al.
2016b).
Ultimately sepiolite, its modified forms and com-
ponents of the recovered waste provides remediation
capability by immobilizing contaminants and restrict-
ing migration in the aqueous phase. This reduces food
chain transfer and may be enhanced by combining
with various additives. However, when considering
the complex characteristics of the soil, the effective-
ness and durability of sepiolite-based materials could
be decreased with low pH (Chen et al. 2020), releasing
pollution and making PTEs bio-accessible to crops
and even increase the risk to humans. Constant
changes in soil environment and soil quality will
continue to present new challenges to application
amount control of products and monitoring by envi-
ronmental laboratories. The use of sepiolite as an
additive for soil remediation may require additional
testing that has not included within regulatory
methods.
Summary, conclusion and future perspectives
This review highlighted the geographical distribution
of sepiolite, its application for PTEs treatment in soil
and land, assessing approaches to modification and
composition on the application of sepiolite-based
systems within site remediation and wastewater treat-
ment. The considerable potential is highlighted for
modified sepiolite or natural sepiolite to be applied in
combination with other treatment methods. These
should not be viewed in isolation, and an integrated
assessment is needed to identify the future influence
on the economic feasibility and if true environmental
control is obtained.
A life cycle approach to sepiolite-based technolo-
gies needs to be developed. Commencing with natural
sepiolite mining and exploitation, the pretreatment,
purification and modification coupled with the waste
generated at each stage. Modified and activated
sepiolites are products that are utilized as soil condi-
tioners, soil amendments or pesticide carriers that
reside in the soil and become integrated part of the soil.
Waste materials from mining exploitation and purifi-
cation could be potentially used alone or combined
with other additives in the treatment of locally slightly
contaminated soil or to be soil fertilizers.
Sepiolite applications can have economic advan-
tages relative to other contamination treatment meth-
ods, such as smaller project costs, more
environmentally friendly impact on contaminated
land, and reduced project risks. The treatment period
of sepiolite applications also compares favorably with
other treatment methods, such as phytoremediation
and constructed wetlands. Sepiolite application com-
bined with other low PTEs accumulating plants has the
potential for cost effective solutions. However, more
investigation is needed to clarify the appropriate scale
of land application, development of technology to
meet that and the potential for regulation change to
allow stabilization and immobilization of the environ-
mental hazard.
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Treatment of environmental contamination using sepiolite: current approaches and future potentialAbstractIntroductionGeographical distribution of sepioliteSepiolite-based material single applicationPolymer nanocomposite sepioliteApplication in machineryApplication in wastewater
Other modified sepiolite materials for treatmentApplication in waterApplication in soil
Sepiolite combined with other additives in treatmentEconomic constraints for the application of sepiolite in remediationDirect project cost and period of treatmentRemediation potential for contaminated landEconomic impacts and project risk
Summary, conclusion and future perspectivesOpen AccessReferences